Simply:
Supply = the amount of coins or tokens in circulation.
There are different types of supply that affect the value of crypto.
Types of supply
1️⃣ Circulating Supply
currently circulating coins/tokens
available to the public
used in the calculation of market cap
2️⃣ Total Supply
the total amount of coins that will ever exist
includes blocked or unsold coins
3️⃣ Max Supply
absolute upper limit on how many coins can ever exist
for example, Bitcoin has 21 million BTC
Why is supply important?